I finally registered here so I can actually use some of the info I have recently learned. I studied vehicle dynamics in school. Duke W and MrPaddleShift have made great points so far. Something no one has seemed to mention is roll steer and chamber change as a result of roll.

First, roll steer. Simply put, when your car leans into a turn (rolls) the suspension geometry changes. Sometimes the kinematics work out so that the inside or outside tire or both may turn a bit adding or subtracting steering angle from the wheels. Most setups try to minimize the roll steer, but if you have any rolls steer I think you would want positive. That is having the wheels add steering angle into the turn under roll. So, if your suspension exhibits roll steer, then a softer anti-sway bar will allow more roll and thus more steer, which makes the car feel less understeer at the steering wheel (notice I say at the wheel, the tire will still be understeering, but is automatically adding steering angle that you can't feel). For a stiffer roll bar the roll steer could be lessened thus seemingly added understeer (at the steering wheel).

Second, camber angle is important. Radial tires are affected less by camber angle than biasply so this effect may be negligible. In a turn the tires also tend to lean in or out, adding or subracting camber (more body roll will create a larger effect, not a linear relationship on most any suspension). In a turn the inside tire will lean in and the outer tire will lean out. This lean creates lateral force in addition to the slip angle of the tire. If you turn your wheels to the limit while stationary and get out, you will see that the tires are leaning. BMW has designed the suspension geometry so that under high lateral acceleration, the tire will still lay flat on the road. The body roll tilts the tire a few degrees the opposite way and it is standing straight up again. By increasing the siffness of the front roll bar you decrease the effect of BMWs steering geometry and remove the lateral force added by the camber change. This will induce understeer.

These are not the largest factors, but just two I saw weren't mentioned. Suspensions are so complex that to tell anything you really have to just test in the real world and at minimum have a full kinematic model of the system. Some of this may be unclear. If it is just ask and I'll try to elaborate.

For those interested, I use Gillespie "Fundamentals of Vehicle Dynamics" for my everyday references. You can find a copy online somewhere for free if you really want one, but I think linking to it would be a violation of rules here. And yes I know this thread is old as balls, but thought I'd add to the forums already huge knowledge base.